Gantry measuring machines are known that comprise a pair of lateral supporting structures, each constituted by a plurality of vertical pillars aligned with respect to one another and by a top horizontal fixed beam supported by the pillars. The fixed beams of the two supporting structures are set parallel to one another and to a first co-ordinate axis of the machine. The machine moreover comprises a horizontal mobile beam set transverse to the fixed beams, and supported by the latter, which can slide in the direction of the first axis along guides provided on the fixed beams. The mobile beam in turn comprises a plurality of guides along its longitudinal axis, that is orthogonal to the first axis and constitutes a second axis of the machine. A carriage, carried by the mobile beam, is movable on the guides of the beam along the second axis. Finally, a column measuring head is carried by the carriage and is movable with respect to the latter vertically along a third axis of the machine. The measuring head is designed to be equipped, at its bottom end, with a detecting device of the contact or remote type.
In measuring machines of the type described, the weight of the mobile members, albeit contained as much as possible, is even so considerable.
As regards the measuring head, which is vertically mobile, it is expedient for the weight to be balanced so as not to load the electric motor driving the head; otherwise, said motor would have to be oversized. In addition, should the weight of the driving head not be balanced, it would be necessary to provide alternative systems for operator safety that can be activated in the case of failure of the motor.
Devices are known for balancing the weight of the measuring head that substantially consist of a single-acting pneumatic cylinder comprising a liner and a piston, which can slide in a fluid-tight way in the liner. The measuring head is fixed with respect to one of the members of the cylinder, for example the liner, whilst the other member, for example the stem, is rigidly connected to a head supporting structure carried by the carriage. By supplying compressed air into the cylinder, it is possible to balance the weight of the measuring head via the pressure force. The head is consequently floating in a vertical direction and subject to the driving forces substantially as if it were weightless.
A drawback linked to known balancing devices is represented by the friction that is generated at the sealing between the piston and the liner, said sealing being generally constituted by a seal housed in a peripheral seat of the piston and sliding within the liner.
On account of the friction, when the head is actuated and therefore the piston slides within the liner, the seal tends to adhere to the liner, and motion unsteadiness phenomena of the type known as “stick-slip” occur, which are problematical as regards the head position control, particularly in the continuous surface scanning operating mode.
The above problems could be solved or attenuated by using high precision cylinders having no seals. However, cylinders of the aforesaid type and of sufficient length for the application are extremely costly and difficult to find on the market.